(1) Field of the Invention
The present invention relates generally to an apparatus for expanding repair sleeves in the tubes of heat exchangers in a nuclear reactor and, more particularly, to an apparatus using an hydraulic expander having multiple bladders and means for accurately detecting the position of a defect within the tube.
(2) Description of the Prior Art
The tubes of heat exchangers such as oil coolers, reheaters or feedwater heat exchangers, in particular those of power plants, are subjected to high mechanical and thermal stresses as well as the corrosive action of the fluid which passes therethrough. The severity of the environment causes degradation of the tubes resulting in cracks. The process of replacing tubes, particularly in power plants where down time must be limited, is both expensive and time consuming. Therefore, it is highly desirable to forestall replacement by repairing localized defects.
It is known to temporarily repair defective tubes by plugging them. However, plugging is only a temporary remedy as it reduces the overall heat transfer surface area of the heat exchanger, and thereby materially reduces its efficiency.
Further, it is known to repair a localized tube defect by installing an inner sleeve which spans the defect. The sleeve is inserted through the end of the tube and thereafter expanded near its ends to form a sealing engagement with the tube. Such expansion may be accomplished by mechanical rolling. Further, in order to secure the sleeve until it is roll expanded, it is known to expand the sleeve by means of a mandrel having an orifice for conducting high pressure fluid and a pair of O-rings flanking the orifice. The O-rings form a sealing engagement with the tube such that as fluid is forced into the space between, the wall of the sleeve is forced outwardly. In order for such mandrel designs to reach the desired expansion pressures, the geometry and surface finish of the sleeve must be tightly controlled. Also, when the expansion process is complete, fluid will leak out of the area between the seals and into the heat exchanger tube. This leakage of water could become a problem if the heat exchanger is, for example, radioactively contaminated. An example of an expander using seals is disclosed in U.S. Pat. No. 5,009,002 to Kelly.
To avoid the problems disclosed above, hydraulic expansion mandrels have been developed which provide one or two bladders for receiving the pressurized fluid. The bladders typically have geometries that are difficult to form. For example, U.S. Pat. No. 4,616,392 and U.S. Pat. No. 4,724,595, both issued to Snyder, disclose a bladder having a lip which engages a groove on the mandrel body for sealing purposes.
U.S. Pat. No. 4,724,693, issued to Tedder, discloses a mandrel wherein the end caps have grooves against which the bladder reacts when pressure is applied. In both cases, sophisticated machining must be performed to assure that the bladders will mate with the other parts of the expander.
U.S. Pat. No. 4,513,497, issued to Finch, discloses a tube expanding technique for securing a sleeve within a tube using a tube expanding device which has a distensible, sealed bladder. The technique utilizes a control system which operates to expand the bladder until the sleeve or tube yield point is detected. After this point, the system is either shut down or an additional volume of fluid is added to bring about a small tube outer diameter increase. It has been found that the technique disclosed does not accurately accommodate variations in the properties of the sleeve and/or the expansion mandrel. In particular, the detected yield point may depend upon the tube and sleeve dimensions and the number and length of the bladders on the expander. Finch recognizes this limitation of the apparatus and technique, stating simply that the use of more than one bladder would result in a decrease in the controllability of the expansion due to the sleeve and tube properties variance between the two points being expanded.
Hydraulic expansion also has been used to expand tubes either within or above and below tube sheets or tube support plates to restrain the motion of either the tube or the plate. Typically, a tube is expanded into the tube sheet to close the Crevice between the tube and the tube sheet. Deposits accumulate within this crevice, creating the risk of tube damage from impurities in the deposits. To minimize the risk of crevice concerns, the tubes are pressurized and expanded until they contact the tube sheet bore. There is little risk of over expanding the tube since the tube sheet provides a very stiff backing to the tube.
A sleeve also may be used to repair a defective portion of a tube not at a tube sheet by expanding the sleeve into the tube to span the defect with a set of expansions on either side of the tube defect. The tube outer diameter expansion size is generally chosen based on qualification testing that takes into account heat exchanger operating conditions and acceptable leak rates as discussed in more detail below. To best control the expansion sizes, it is not possible to use pressure as a means of controlling the process due to potential variations in the yield strength and size of the tube and sleeve material. In the free-span portion of the tube, where it is not backed by the tube sheet material, the risk of over expanding the tube is great if the expansion process is not tightly controlled. If an overexpansion occurs, the tube may crack either during the expansion operation or during the operation of the heat exchanger due to high stresses in the expanded region.
The acceptable leak rate between the sleeve and tube is an issue within all designs of heat exchangers. However, in radioactively contaminated heat exchangers, the leakage from the contaminated side of the tube to the clean side will result in the contamination of otherwise clean system components. The heat exchangers with dissimilar fluids on each side of the tube, such as an oil cooler, where one side of the tube is filled with water and the other with oil, leakage will result in the mixing of one fluid with the-other, a potentially unacceptable situation that may result in damage to other system components. Finally, in pre-heater or feedwater heat exchangers, the leakage of fluid from one side of the tube to the other will result in the loss of efficiency of the unit. Based on the type of heat exchanger to be sleeved, an acceptable leak rate for the sleeve can be determined.. In cases where the flow rate through the heat exchanger is high and the risk of contamination or damage to the system components is low, a higher leak rate may be acceptable.
In order to properly locate a sleeve, it is important to accurately locate the center of the sleeve with respect to the defect in the tube. U.S. Pat. No. 4,159,564, issued to Cooper, Jr., discloses the use of an eddy current coil to detect the presence of the end of the tube sheet. Because the device disclosed uses the tube to return the eddy current signal to a monitoring device, there is a significant amount of electrical noise due to both stray currents within the tube and the electrical connection between the coil and the tube. With this high electrical noise, it is not possible to detect small tube defects. Moreover, in a typical heat exchanger, the inside of the tubes are typically coated with a layer of oxides or other deposits, creating a non-uniform electrical connection. The eddy current device described in the patent to Cooper, Jr., could only reliably be used where the tubes were cleaned prior to sleeving, a man-power intensive effort.
Thus, there exists the need for a hydraulic sleeve expander for installing sleeves in the tubes of a heat exchanger which dispenses with the need for mechanical rolling and other operations for sealing the ends of the sleeves. Further, there exists a need for such an expander which is versatile and relatively inexpensive to manufacture while, at the same time, is capable of accurately locating a defect in a tube and positioning a sleeve relative to the defect.